This is a video described as “Kitten Talks Too Much“. The cat is looking out of the window and chattering. This cat is not talking. The noise this cat makes is not a form of communication. Although we can’t see what she sees, it is very likely to be birds. She is doing the next best thing to preying on an animal. She is opening and closing her jaw rapidly to apply a bite to the nape of the neck of the prey while at the same time trilling and making excited sounds.
This is a form of practice for killing prey with a bite to the upper spine to break the neck; or the cat killing equivalent of playing air guitar. Her tail is also swishing from side to side indicating excitement and uncertainty as to what to do next – a result of the competing forces of wanting to hunt but being unable to.
This cat is almost certainly an indoor cat. This is as near as she gets to expressing that most fundamental of drives: hunting and killing animals.
This page: cat sounds, has another video of a cat chattering. Chattering takes various forms depending on the individual cat. I am sure we have all seen it. All indoor cats who can look out of a window might chatter like this depending on how active their residual drive to hunt is.